A tumor-host system with unique features of metastasis in puppies but with spontaneous regression after 2 to 5 months of progressive growth in adult non-immunosuppressed normal dogs will be sued to assess the conditions of the host-tumor relationshi which are responsible for progressive tumor growth and spontaneous regression. The role of circulating (shed) tumor antigen and immune complexes on universal take of the tumor in allogeneic hosts, subsequent progressive tumor growth, and spontaneous regression after 2 to 5 months will be evaluated and related with the immune reactivity of the tumor host. Extraccorporeal immunoadsorption will be used also to evaluate the effect of antigen and immune complex reduction on tumor behavior. Monoclonal antibodies specific for subsets of canine lymphocytes will be prepared and the identity and functional role of subpopulations of lymphocytes infiltrating the tumor mass at different stages of growth will be determined in order to identify the cells which are responsible for spontaneous regression of the tumor. The rejection process, once started, generally completes within 2 to 3 weeks and the animals become immune to subsequent tumor challenge. An understanding of the mechanisms responsible for this phenomenon which occurs both in nature and in the laboratory, is of utmost importance in tumor and transplantation immunity. In addition, the mechanism of matero-offspring transfer of tumor immunity previously observed in our laboratory will be studied in the offspring fed or deprived of colostrum.